There are known machine tools of the type in which a plurality of tools may be exchanged one by one automatically, as shown, for example, in U.S. Pat. No. 3,789,473 and British Pat. No. 1,333,191, and another machine tools of the type in which a plurality of gang heads each having plural tools may be exchanged one by one, as shown, for example, in U.S. Pat. No. 4,110,898 and Japanese Patent Publication No. 47,942/78. However, there has not been known a machine tool in which a plurality of gang heads as well as a plurality of tools may be exchanged one by one automatically.
There are known machine tools in which a magazine for storing a plurality of gang heads is provided above a liftable drive unit which is selectively lifted for a quick exchange of gang head to be equipped to such drive unit. In the machine tools of this type according to the prior art, a gang head is clamped only at the input shaft thereof to a drive unit or the spindle of such drive unit. In such machine tools, relatively small-scaled gang heads are employed as exchangeable heads for securing a required clamping strength.
It is preferred for a high working efficiency to employ large-scaled gang heads each performing a working on a number of portions or a large area to be worked at a same time.
However, a machine tool according to the prior art in which a liftable drive unit is employed for a quick exchange of gang head provides a limit in enlarging the scale of gang head from the clamping strength therefor referred to above. Further, a large-scaled gang head will cause a problem that it is apt to leave unworked small portions of a workpiece to be worked.
Accordingly, a primary object of the present invention is to provide a novel machine tool of the type having a liftable drive unit for a quick exchange of gange head in which exchangeable gang heads may be large-scaled for a high working efficiency and in which a plurality of tools for working small portions of workpieces which are unworkable by a large-scaled gange head may be exchanged quickly, too.